AS guide: documents Object#subclasses_of

railties/guides/source/active_support_core_extensions.textile

@@ -329,6 +329,40 @@ h5. +remove_subclasses_of+
 
 The method +remove_subclasses_of+ receives an arbitrary number of class objects and removes their subclasses. It is a wrapper of +Class#remove_class+ explained with more details in "Class Removal FIX THIS LINK":FIXME.
 
+h5. +subclasses_of+
+
+The method +subclasses_of+ receives an arbitrary number of class objects and returns all their reachable descendants as a single array:
+
+<ruby>
+class C; end
+subclasses_of(C) # => []
+
+subclasses_of(Integer) # => [Bignum, Fixnum]
+
+module M
+  class A; end
+  class B1 < A; end
+  class B2 < A; end
+end
+
+module N
+  class C < M::B1; end
+end
+
+subclasses_of(M::A) # => [N::C, M::B2, M::B1]
+</ruby>
+
+The order in which these classes are returned is unspecified. The returned collection may have duplicates:
+
+<ruby>
+subclasses_of(Numeric, Integer)
+# => [Bignum, Float, Fixnum, Integer, Date::Infinity, Rational, BigDecimal, Bignum, Fixnum]
+</ruby>
+
+See also +Class#subclasses+ in "Extensions to +Class+ FIXME THIS LINK":FIXME.
+
+NOTE: Defined in +active_support/core_ext/object/extending.rb+.
+
 h4. Instance Variables
 
 Active Support provides several methods to ease access to instance variables.